1. Field of the invention
The invention relates to a fuel vapour storage and recovery apparatus for the fuel system of a motor vehicle comprising a vapour storage canister containing adsorbent material, which is intended to trap and store vapours from the vehicle's fuel tank, and to purge stored vapour and condensate during operation of the vehicle's engine.
2. Discussion of the Background
Fuel vapour escaping from the fuel system is a source of hydrocarbon emission from the automobile. In particular, gasoline vapours may escape from the external vents of the fuel tank, either while driving or while at rest. Within the past several years a number of attempts have been made to limit the escape of gasoline into the atmosphere.
There have been proposed evaporative emission control systems that involve the use of charcoal-filled canisters which are connected, through vapour lines, to the fuel tanks such that gasoline vapours from the tanks are channelled into the canisters, and adsorbed or partially condensed in the charcoal. The vapour line in such a system is connected to an inlet port located at the top of the canister. A second port on the canister is also provided, known as a “purge” port, from which a line extends to the intake manifold or carburettor of the vehicle's engine. When the engine is running, at a preset frequency, condensed fuel and vapour that is stored in the charcoal is released and sucked into the engine, to be burned.
A major problem with evaporative emission control systems of the type employing activated carbon or activated charcoal as adsorbent materials housed in a vapour canister, is that under conditions of low ambient temperature, the fuel that has been adsorbed exhibits a reluctance to be released and purged from the canister. It has been determined that with adsorptive substances currently being employed, satisfactory release of the stored fuel will occur at room temperatures and above; however, when the temperatures fall much below these values, the efficiency of the system suffers significantly. The ability of the carbon to release the fuel is poor until the canister temperature rises.
Therefore, evaporative emission control systems have been proposed in which the temperature of the adsorbent material is regulated by an electrical heating system. In this regard it can be referred e.g. to U.S. Pat. No. 4,598,686, U.S. Pat. No. 4,721,846, U.S. Pat. No. 4,778,495, U.S. Pat. No. 4,864,103, U.S. Pat. No. 6,230,693, EP-A 905 368 and GB 2 329 217. However, electrical heating systems are disadvantageous as they require costly technical equipment and additional energy consumption.
Alternatively, it has been proposed to control the temperature of the adsorbent material by the waste heat produced by the internal combustion engine. Accordingly, evaporative emission control systems have been developed in which the vapour storage canister is located in the proximity of the engine, i.e. in the engine compartment. Other systems rely on passively heating the adsorbent material by hot air which has absorbed heat from the engine before it enters the vapour storage canister. Various heat sources for heating the air have been proposed among which the engine itself, the cooling water circuit and the engine exhaust system. In this regard it can be referred to e.g. U.S. Pat. No. 3,093,124, U.S. Pat. No. 3,221,724, U.S. Pat. No. 3,757,753, U.S. Pat. No. 4,829,968, U.S. Pat. No. 4,846,135, U.S. Pat. No. 5,054,453, U.S. Pat. No. 6,098,601 and U.S. Pat. No. 6,698,403.
In the evaporative emission control systems of the prior art, however, the air for purging the fuel vapour adsorbent material is heated in the proximity of the exhaust pipe, the cooling water circuit or the engine compartment and is then conducted to the adsorbent material. Since it is disadvantageous and possibly dangerous to conduct the fuel vapours over a long distance, usually the fuel vapour adsorbent material is in a container which is close to the fuel tank or is an integral part of the fuel tank, which is generally far away from the heat source. Therefore, the hot air must be conducted from the heat source to the adsorbent material which is close to the fuel tank over a long distance. This requires a complex and space consuming conduit system. Furthermore, the relatively long conduit must be properly insulated in order to avoid significant cooling of the hot air until it reaches the adsorbent material.
On the other hand, fuel systems are known wherein the fuel tank is close to the exhaust pipe and protected from the heat radiation emitted there from by means of a heat shield. Said heat shield is located between the fuel tank and the exhaust pipe in order to avoid heating of the fuel tank and of the fuel contained in the fuel tank above a certain temperature.